External skeletal fixator systems are used on fractured bones or joints during osteosynthesis typically for more serious injuries involving multiple or compound fractures. Pins are drilled through a patient""s skin and into a fractured bone. Clamps secure the pins to a common connecting rod, creating a rigid frame that immobilizes the bone.
The Kirschner-Ehmer external fixator system is commonly used in veterinary orthopedic surgery. The system includes: a standard connecting rod; clamps adapted to slide over the ends of the connecting rod; fixator pins; and bolts for tightening the clamps around the connecting bars, which bolts have through-holes near the head for receiving the fixator pins"" shafts.
The Kirschner-Ebmer system precludes installation of an additional clamp between two installed clamps on the connecting bar or removal of an intermediate clamp without disassembly of the entire connecting rod. This is because the Kirschner-Ehmer clamp includes a narrow slot leading to a wider channel, into which channel a connecting rod is inserted axially. The connecting rod will not fit into the slot to allow for transverse mounting. Consequently, the surgeon must anticipate the number of clamps required and slide them onto the connecting rod before insertion of the end pins into the bone. This is especially limiting if an additional pin or clamp is required at the completion of surgery or at subsequent patient visits. In addition, the Kirschner-Ehmer clamp undergoes plastic deformation when tightened, permanently deforming and fatiguing the material. Consequently, reuse of the clamp is discouraged. Furthermore, the Kirschner-Ehmer clamp is inapplicable in ring fixators such as the Ilizarov external ring fixator.
A fixator clamp produced by Synthes(copyright) permits transverse installation of a clamp on a connecting rod between two installed clamps without rod disassembly. It includes a slot for receiving a connecting rod, a hole for receiving a pin in an orthogonal direction relative to the connecting rod, a clevis-shaped clamp, and a bolt and nut which simultaneously secure the pin in the hole and the clamp on the connecting rod. This design is limited to orthogonal pin placements relative to the connecting rod and is mechanically complex.
The Synthes(copyright) and Kirschner-Ehmer designs are particularly susceptible to loosening under repeated cyclic loading. The Kirschner-Ehmer device relies entirely on the axial tension in the bolt to preclude pin rotation. A slight amount of loosening due to cyclic loading reduces axial tension in the bolt, allowing the pin to rotate relative to the connecting rod. The Synthes(trademark) design relies on a clamp that touches the connecting rod at only two points, rendering this device susceptible to loosening.
Improved external fixator clamp designs have been described in U.S. patent application Ser. No. 08/643,512, filed May 6, 1996, entitled xe2x80x9cExternal Fixator Systemxe2x80x9d, filed by Kraus, et al., the teachings of which are incorporated herein in their entirely by this reference. There, the clamp body had a slot that was wide enough to transversely receive and snap-fit over the connecting rod. A hook-shaped bolt, rather than the eye-bolt of the Kirschner-Ehmer designs, was used to hold the fixator pin while securing the clamp body to the rod. This configuration enabled the clamp body to be conveniently added in the middle of a rod.
The present invention is directed to an external fixator clamp that incorporates a number of advantages achieved by the designs described in 08/643,512 application. The present clamp body has a slot that is wide enough to also transversely receive the rod. Innovations, however, are introduced that relieve a number of tolerances in the manufacture of the clamp, thus substantially reducing the per unit cost, while simultaneously increasing the clamp""s rigidity.
Rather than a relatively long bolt to secure the pin to the clamp, a pin connector is used that is inserted into a connector bore in the clamp body. This connector has a pin bore for receiving the fixator pin and a rod-engaging surface. The connector is urged into the connector bore until the fixator pin is braced against the clamp body and the rod-engaging surface wedges the connecting rod into the slot to ensure that the rod is seated in the back of the slot. The interference between the connector and rod provides increased resistance to the rotation of the pin relative to the clamp body and rotation of the clamp body relative to the rod. Moreover, since the rod will always be fully seated in the slot by the connector, the clamp body need not snap-fit onto the rod. The snap-fitting functionality required precise manufacturing tolerances in the machining of the slot since the clamp was generally rigid, allowing for only slight amounts of flexing under reasonable forces. In contrast, in the present invention, the slot is preferably machined to the width of the rod.
In the preferred embodiment, a bolt is inserted into a distal end of the connector bore to mate with threads of the pin connector to thereby draw the pin connector into the connector bore. Alternatively, the connector could be externally threaded, but in either case, the connector should have a short overall length to reduce the amount of clearance necessary to couple the connector to the clamp. That is, when the clamp is constructed, the connector extends only partially across the width of the clamp body, the remainder of the distance being covered by the distally inserted bolt or similar fixture.
In general, according to another aspect, the invention also features an aiming device and method to assist in the installation of fixator pins. The device has a frame that is adapted to be attached to first and second connecting rods, between which the pin is to be connected. A guide is provided on the frame to facilitate the pin installation process.
In specific embodiments, the guide may be a bore in the frame or alternatively a sleeve, which is made part of the frame. This guide may provide for pin installation at various angles. The frame is preferably adjustable so that clamps for the pins can be installed either above or below the rods. The frame is also preferably expandable to accommodate various distances between the connecting rods.
The above and other features of the invention including various novel details of construction and combinations of parts, and other advantages, will now be more particularly described with reference to the accompanying drawings and pointed out in the claims. It will be understood that the particular method and device embodying the invention are shown by way of illustration and not as a limitation of the invention. The principles and features of this invention may be employed in various and numerous embodiments without departing from the scope of the invention.